Photopolymerizable recording materials are known for use in preparing printing plates for flexographic printing. Such materials are used to make printing plates by first exposing imagewise a light-sensitive layer followed by removal of the unexposed areas. In this art area, solid, photopolymerizable materials are differentiated from those in which the imagewise exposure is conducted in liquid systems. In addition, there are photopolymerizable recording materials that are aqueous-developable and those that are organic-solvent developable.
EP-A2 02 61 890 (U.S. Pat. No. 4,857,434) discloses an aqueous-developable, liquid system using liquid hydrocarbon/maleate polymers, soluble in an aqueous medium, with (meth)acrylic acid groups and maleic acid half ester groups linked through ester groups.
Aqueous-developable printing plates are also described in DE-A1 25 09 842. The plate contains a binder of a copolymer that is soluble in aqueous alkaline solutions of unsaturated carboxylic acids and hydroxyalkyl (meth)acrylates, and which can also react with, among others, cyclic acid anhydrides to half esters. Similar binders with added amino or ammonium groups are also described in EP-A3 02 06 412 (U.S. Pat. No. 4,734,476), for preparing color filters, that is, after exposure and washoff, the layers are permanently colored with dye solutions.
There are several disadvantages to using liquid systems. First, liquid systems require commercially expensive processing. In addition, compared to organic-solvent developable printing plates, aqueous-developable printing plates have low stability to alcoholic and aqueous printing inks. Such low stability causes swelling of the printing surface which results in defective printing.
Examples of solid, solvent-developable recording materials are disclosed in the following patents: DE-C3 22 15 090 (U.S. Pat. Nos. 4,323,636; 4,323,637; 4,369,246; 4,423,135); U.S. Pat. No. 4,266,005; U.S. Pat. No. 4,320,188; and U.S. Pat. No. 4,430,417. Photopolymerizable printing plates prepared from solvent-developable recording materials usually comprise a support, an optional adhesive or other undercoating, a photopolymerizable layer having at least one polymeric binder, at least one ethylenically unsaturated monomer addition-polymerizable by actinic light, and a photoinitiator or photoinitiator system, and a cover element consisting of a flexible, polymeric film soluble in the developer solution and a separable cover sheet.
A preferred method for preparing printing plates from such multilayer, photopolymerizable elements is a process in which a previously extruded photopolymerizable composition is fed into the nip of a calender and calendered between a support and a cover element to form a photopolymerizable layer between them.
EP-B1 00 84 851 (U.S. Pat. Nos. 4,427,759 and 4,460,675), discloses preparing a multilayer, photopolymerizable printing element wherein an elastomeric layer is interposed between the flexible polymeric film of the cover element and the photopolymerizable layer. The elastomeric layer contains an elastomeric binder, and as optional constituents, a second binder, a dye, one or more monomers, and a photoinitiator.
A disadvantage to preparing solvent-developable flexographic printing plates from the above-disclosed materials is that the plates produced have uneven printing quality. Unsatisfactory printing results are obtained with alcoholic and aqueous flexographic printing inks, because, inadequate ink transfer in printing leads to ink defects. The transfer of 5% dots is particularly prone to defects in the use of such printing inks.
Accordingly, the object of the present invention is to eliminate the printing difficulties discussed above and make available solid, photopolymerizable printing elements for preparing flexographic printing plates having improved ink transfer. In addition, conventional washoff development with organic solvents should be feasible. At the same time, other properties of the photopolymerizable printing plates should not be affected adversely. For example, a reduction in photosensitivity should be avoided and the flexographic printing plates should not swell in printing inks, because undesirable dot growth and consequently, defective printing results are obtained.
Surprisingly, the above-mentioned objectives are achieved using plates prepared from a photopolymerizable printing element comprising (a) a support, (b) a photopolymerizable layer having at least one elastomeric binder, at least one ethylenically unsaturated monomer addition-polymerizable by actinic radiation, and a photoinitiator or photoinitiator system, (c) an elastomeric layer having at least one thermoplastic, elastomeric block copolymer, and (d) a cover sheet.